The present invention generally relates to a circuit for displaying screen control states of a monitor, wherein a video signal output from a video card is maintained to be equal to a voltage level of an on-screen-display (referred to as OSD hereinafter) signal by using a horizontal flyback signal, and the video signal and the OSD signal are controlled not to be overlapped with each other by using a blank signal.
In general, most monitors include light emitting diodes (referred to as LED hereinafter) for indicating (or displaying) function control states upon controlling the function keys to control screen states.
However, the monitors using the LEDs need a large space for arranging the LEDs thereon.
Further, since the LED display indicates only a selected screen mode, it can not display the screen control states finely, giving inconveniences in displaying the functions performed by more than two keys.
Accordingly, to solve these problems, Korean patent application No. 92-7980 discloses a technology for display screen control modes and screen control states selected by the key operations on the screen by using the OSD function.
That is, as shown in FIG. 1, when OSD letters are not displayed on the screen, R(red), G(green), B(blue) signals supplied from a video card 11 are applied to a pre-amplifier 15 through a mixer 14 to be amplified to a predetermined voltage level. The pre-amplified signal outputs from the pre-amplifier 15 are again inversely amplified to a voltage level suitable for driving a CRT (Cathode Ray Tube) and then applied to cathode electrodes of a CRT 17.
Alternatively, when the OSD letters are displayed on the screen, a blanking signal BLK and the R, G, B signals are supplied simultaneously from an OSD IC (integrated circuit) 12. At this moment, the blanking signal BLK is at the logic "high" state and transferred to a transistor Q1 through a diode D1 and a resistor R1, thus for turning on the transistor Q1. If the transistor 01 is turned on, a voltage level at a collector thereof becomes lower. Thus, while the OSD letters are displayed, the R, G, B signals supplied from the video card 11 are muted from an area on which the OSD letters are to be displayed.
Accordingly, the R, G, B signals supplied from the OSD IC 12 are applied to the pre-amplifier 15 through the mixer 14 and then pre-amplified to a predetermined voltage level. The pre-amplified signal outputs from the pre-amplifier 15 are again inversely amplified to a voltage level suitable for driving the CRT 17 by the amplifier 16 and applied to the cathode electrodes of the CRT 17 to display the OSD letters on the screen.
However, when adapting the above OSD device to a monitor, a manufacturer may encounter the following problems.
First, data displayed on the screen is generally a mixture of texts and complicated graphics. In this case, since the blanking signal BLK and the R, G, B signals are simultaneously supplied from the OSD IC 12, the R, G, B signals from the OSD IC 12 are applied to the mixer 14 before the blanking signal BLK from the OSD IC 12 mutes the R, G, B signals from the video card 11. Therefore, the R, G, B signals from the video card 11 are overlapped with the R, G, B signals from the OSD IC 12 at the beginning. Thus, the OSD letters are overlapped with the complicated graphic or text from the video card 11, thereby making a user confusing.
Second, the R, G, B signals supplied from the video card 11 are generally an analog signal lower than 1VP-P. Thus, when an additional circuit such as the mixer 14 is provided at a pre-stage of the pre-amplifier, the characteristics of the input signals are degraded.
Third, since the R, G, B signals supplied from the video card 11 are generally an analog signal lower than 1VP-P and the R, G, B signals from the OSD IC 12 are 5 voltage, the voltage level of R, G, B signals from the OSD IC 12 must be lowered to the level of the R, G, B signals from the video card 11 so as to maintain the voltage level of the R, G, B signals from the OSD IC 12 at the same level as that of the analog input voltage from the video card 11. Accordingly, the characteristics of the OSD display letters become degraded.